Move file stuff back to plFile for gcc.
Add UnicodeBuffer as first class data type and allow plStrings to be constructed with it
Clean up some useless junk from plString
Use plString in plConfigInfo and friends
Remove xtl::istring and xtl::iwstring
Remove an unnecessary extra heap allocation in plString::Format
Replace xtl::delete_ptr functor with lambdas
Remove xtl::copy_if because:
- It is included in C++11
- Nobody was using it anyway >_<
Add identification for some more modern Windows versions
Remove unused headers from hsStlUtils.h (and push them downstream where needed)

Also fix a couple of plString and header issues identified by Paradox on GCC/Clang
Deprecate and remove xtl::format in favor of plString::Format
Fully remove hsStlUtils, as it is now completely unused
Deprecate and remove hsStlSortUtils